Since the seventies, when optical telecommunications based on sufficiently low attenuation were developed to allow the propagation of light to distances longer than 1 km, huge progress has been made in order to develop at the same time techniques using optical fibers designed for the measurement of a wide variety of physical, chemical and even biological quantities.
The main reasons for these efforts are some features inherent to optical fibers such as low weight, flexibility, long transmission distance, low reactivity, electrical insulation and electromagnetic immunity. Besides, there is in many cases the possibility of multiplexing the signals of several sensors, including those directed to the measurement of different quantities and even the possibility of carrying out measurements continuously distributed along the sensing fiber.
Optical fiber sensors are, therefore, optical sensors utilizing fibers as a connecting means for light between the mensurand and the measurement area. Optical fiber sensors can be divided between extrinsic and intrinsic. In the first category are those where the fiber is only a waveguide and the optical effect to be measured occurs out of the fiber. In the second case the fiber is also a medium where the coupling between the mensurand and the light occurs, this rendering this kind of sensor more practical under the mechanical point of view. Sensors can also be considered as for the kind of optical effect to be measured, such as a change in intensity, in polarization, in the spectrum or in the phase of the light wave.
Since the nineties a new component is becoming more and more important not only in telecommunications but also in various applications in the sensor field. This component, called fiber Bragg grating (FBG) is nothing more than a reflective optical fiber having an extremely high spectral selectivity. Its setup is based in the generation of a periodical modulation in the refractive index of the fiber core, such structure being able to efficiently reflect the λb wavelength satisfying the first order Bragg condition for normal incidence, that is, equation (1) below:λb=Λ/2n  (1)where Λ is the spatial period of the index modulation and n is the refractive index of the fiber.
The sensing ability of the Bragg gratings is related to the fact that λb can be altered by mechanical efforts modifying the structure periodicity, Λ, or through temperature that modifies the refractive index n. Such dependencies can be approximately summarized in the expression of equation (2) below:Δλb/λb=9×10−6ΔT+0.78ε  (2)Where ΔT is measured in degrees Centigrade (° C.) and ε is dimensionless (m/m). The numerical constants are those typical of the material of which the fiber is made up, and particularly, the thermal constant, can vary depending on the fiber.
The information provided by FBG's is contained in spectra, which renders the measurement an absolute, easy to multiplex quantity and makes Bragg gratings particularly attractive for use in sensors.
For usual wavelengths (1300 nm and 1550 nm) equation (2) implies that the measurement of λb should be performed at an accuracy of the order of 1 pm in order to obtain an accuracy of 1 ppm (1 μm/m) strain or 0.1° C. in temperature. There are several ways of reaching this objective, as will be seen below.
Based on the modifications brought about in the fiber Bragg grating optical spectrum of reflection, different procedures can be employed for the measurement of strains or temperature changes. The choice is not an obvious one and chiefly depends on the desired application, and for each case one must consider the frequency bands involved, the number of interrogated sensors, their spatial distribution, the dynamic range of strains or temperatures to be measured, space and weight limitations imposed by the measurement system and, for sure, the cost involved.
Some of the most widely used techniques for the measurement of Fiber Bragg grating sensors are those that use adjustable band filters for the sweeping of the optical spectrum. To this context belongs the simplest technique consisting in the direct measurement by means of an Optical Spectrum Analyzer (OSA).
It is possible to obtain a resolution in the 1 pm range in the spectrum measurement, which corresponds to strains close to 1 μm/m or 0.1° C. temperature changes. The dynamic range for strain measurements is limited according to the number of interrogated sensors. By using two LEDs in the typical wavelengths of 1300 nm and 1550 nm, the relationship between the dynamic range, Δε, and the number of sensors, N, can be estimated by means of the relationship below (equation (3)):Δε=105/N (in μm/m)  (3)
Thus, for example, for the measurement of one hundred sensors using a commercial OSA, the dynamic range estimated for each measurement site is of the order of 1,000 μm/m. The main advantage of using a general purpose OSA is the simplicity and quick setting of the measurement system. However, the high cost of the equipment should be considered as well as the slowness at which is carried out the sweeping of the optical spectrum—typically, one sweeping per minute at a range of 100 nm, which practically restricts its use to static measurements. Therefore such technique should in general terms be considered for situations where the measurements are not permanent so that the equipment can be employed for additional applications. In case it is desired to measure a huge number of sensors in the same fiber, such alternative can become economically attractive.
Further, it should be pointed out that such equipment can be utilized as a fixed filter system, similar to that which will be discussed later on in the present specification. This way of utilization allows obtaining dynamic measurements (a few hundreds of Hz) but certainly should not be used in a continuous mode in view of the equipment cost. Finally, a very interesting feature is the easy calibration that can be performed, either continuously or periodically, by introducing a wavelength reference that can be made up of a gas cell or a Bragg grating in a thermally compensated encapsulation.
Fabry-Perot filters having the spectral band determined by a cavity that can be dynamically altered by for example, piezoelectric actuators, are also used for performing spectral sweepings. In an optical circuit that can be employed together with an adjustable pass band Fabry Perot filter the pass band is made to vary in an alternate way through linear slopes, so that each of the interrogated sensors is sequentially illuminated. Resolutions close to 1 μm/m can also be obtained through the use of this technique. The sweeping of such filters is typically limited to nearly 100 nm, the frequency response hardly being higher than a few tens of Hz.
Analogously to the previous case, the cost of this technique does not depend on the number of sensors to be interrogated, this rendering it more competitive as the number of measured sites is increased.
For systems having a not very large number of sensors, a lower cost alternative employs fixed spectral filters. Such filters can be of the Fabry-Perot kind, Mach Zender interferometer, or even a Bragg grating as in the case of the present invention. FIG. 1 attached shows optical circuits using this technique. The system employs broadband sources and the signal reflected by the grating used as sensor is directed, through a 3 dB coupler, to the filter and to a reference detector. The optical signal resulting from the interaction with the filter is then guided towards the other detector, and its electrical outlet it divided by the one obtained by the reference detector. The utilization of a reference signal aims at compensating fluctuations in the optical source. As explained in more detail hereinbelow, it was experimentally found that the proposed implementation allows the measurement signal to be kept stable, with a change lower than ±0.5% while the power supplied by the LED is reduced in up to 30%. The topologies proposed for the measurement of four sensors, illustrated in FIG. 1 attached to the present specification, can be sufficiently extended to up to 16 sensors without any apparent technical problems. The utilization of two sources makes the system more robust. The cost for implementing the solutions proposed in FIG. 1 is rather low for the measurement of just one sensor when compared to the acquisition of previously described equipment. As more channels are added to the system such cost increases linearly.
Besides the modularity, another important advantage in the utilization of fixed spectral filters is the possibility to apply such devices in dynamic measurements. The frequency range is limited by the photodetector's response and can easily reach a few hundreds of kHz. The computational modeling of the reflection of a broadband optical signal by the sensor and then by the filter means that smaller uncertainties are obtained by using two gratings (sensor and filter) having identical spectra. Uncertainty and resolution are dictated by the photodetector frequency response. Based on simulated data it is possible to estimate that for measurements in a 10 Hz band, uncertainties of ±0.1% would be obtained in a dynamic range of ±1,500 μm/m.
A further set of procedures potentially applicable to the measurement of systems requiring the interrogation of several sensors is that based on time multiplexing.
One possibility in this area consists in the utilization of an OTDR—Optical Time Domain Reflectometer. The sensing gratings, which can be written in a same wavelength and in a same optical fiber, should bear low reflectivity, of the order of 1%. However, it should be pointed out that in view of the working principle of an OTDR the utilization of this technique is limited to static measurements.
Among the above-mentioned techniques, doubtless the fixed filter system is the cheapest available for a small number of sensors and it is also the system having the quickest response, with the possibility of reaching several kHz according to the electronic system. Thus, the surface data acquisition system disclosed hereinbelow in the present specification employs a fixed filter system where the filters are made up of Bragg gratings.
U.S. Pat. No. 5,401,956 teaches a practical diagnosis system working in cooperation with remote optical fiber sensors containing Bragg gratings for measuring static strain, dynamic strain and/or acoustic/vibratory perturbations of items or structures.
U.S. Pat. No. 5,426,297 teaches a system allowing a plurality of Bragg grating sensors in one single fiber as well as in a plurality of fibers, each one having a plurality of Bragg gratings to be detected, such system detecting each of the wavelengths and shifts of the same reflected by the Bragg grating.
U.S. Pat. No. 5,493,390 teaches a system involving a source of light, an optical fiber containing a Bragg grating forming a sensor reflecting a wavelength in response to a perturbation, integrated tunable opto-acoustical filter placed in the path of the light emitted by said sensor for filtering the light received from the sensor, the filter pass band being adjustable to superimpose to the reflection wavelength of the sensor in response to a control signal of the filter, and to provide a filtered signal the power of which is related to the optical transmission; optical detection device for detecting the power of the filtered signal and providing a detection signal and a device for signal processing in response to the detection signal for providing the filter control signal, detecting a shift in the reflection wavelength caused by the perturbation, the signal processing device including devices for adjusting the filter control signal to follow static shifts in the reflection wavelength and dynamic shifts in the sensor wavelength caused by static and dynamic shifts in the perturbation, for a predetermined length of time, and for providing output signals able to indicate the static and dynamic shifts in the perturbation.
In spite of the approaches provided for by the state-of-the-art technique, there is still the need of a system for the measurement and surface data acquisition for fiber Bragg grating-based optical fiber pressure and temperature sensors, said system comprising: i) an optical system for signal processing with an optical source transmitting a signal through an optical coupler, said signal being conveyed to fiber Bragg grating (FBG) optical fiber sensors, the optical signals returning from said optical fiber sensors passing by couplers and divided in outputs so as to yield reference signals conveyed to detectors; ii) an electronic signal processing system, and iii) an optical switcher with an interface, connecting the optical fiber sensors containing Bragg gratings for the measurement of physical parameters such as pressure and temperature in an oil and/or gas well and the optical and electronic systems, such system for the measurement and data acquisition being described in the present application.